Dairy cows are significant investments for dairy farmers, and enormous efforts, such as animal breeding and artificial insemination, have been and continue to be invested in breeding programs to improve the animals. Typically, for unknown reasons, artificial insemination in dairy cattle is successful only 30-35% of the time. However, it is understood that both biological and environmental factors affect fertility rate. Some environmental factors such as heat and lack of precipitation, can cause stress in cattle and can decrease the fertility rate to 10-15%. Commercial artificial insemination operations often shut down in July and August due to the drop in fertility caused by the hot, dry weather. It is also known that certain bulls are more fertile than others due to their genetic makeup. Identifying highly fertile bulls, however, is a time consuming and expensive process. It can take 5-10 years of tracking the attempts of artificial insemination using semen from the bulls before they can be certified as quality bulls.
There is thus a need for a method of genetically evaluating the bulls, e.g., by genetic testing, to enable a quick and accurate evaluation of its fertility as well as the survival rate of embryos conceived therefrom. Genetic testing of the bulls to determine their fertility and embryo survival rate can lower the high cost of the traditional, progeny testing methods, by-passing the need to produce live birth.
There is further a need to ensure that the dairy cattle have highly desirable productive traits, such as milk fat content and protein content. In this regard, traditional breeding techniques involve the studying of sire progenies, and evaluating their traits including milk production ratings (transmitting abilities) to guide further breeding. This standard technique is similarly time consuming and costly, requiring years to evaluate the true genetic value by progeny testing of each bull. Many cows must be bred and give birth to offspring. The females must be raised, bred, allowed to give birth and finally milked for a length of time to measure their phenotypic traits. Furthermore, selection based purely on phenotypic characteristics does not efficiently take into account genetic variability caused by complex gene action and interactions, and the effect of the environmental and developmental variants. There is thus a need for a method of genetically evaluating cattle to enable breeders to more accurately select animals at both the phenotypic and the genetic levels.
Marker-assisted selection can lower the high cost of progeny testing currently used to improve sires, since young bull progeny could be evaluated immediately after birth or even before birth, and those young bulls that are determined by genetic testing to have undesirable markers would never be progeny tested, for the presence/absence of the marker. Therefore, there is also a need for genetic markers for such marker-assisted selection process.
The signal transducer and activator (STAT) proteins are known to play an important role in cytokine signaling pathways. STAT proteins are transcription factors that are specifically activated to regulate gene transcription when cells encounter cytokines and growth factors, hence they act as signal transducers in the cytoplasm and transcription activators in the nucleus (Kisseleva et al., 2002). In mammals, STATs comprise a family of seven structurally and functionally related proteins: STAT1, STAT2, STAT3, STAT4, STAT5A and STAT5B, STAT6 (Darnell, 1997). The seven mammalian STAT proteins range in size from 750 to 850 amino acids. The chromosomal distribution of these STATs, as well as the identification of STATs in more primitive eukaryotes, suggest that this family arose from a single primordial gene (Chen et al., 1998). In addition, STATs share a number of structurally and functionally conserved domains.
The STAT5 protein is also known as the mammary gland factor. This protein was initially identified in the mammary gland as a regulator of milk protein gene expression (Watson, 2001). STAT5A is a member of the interferon-tau (IFN-tau) and placental lactogen (PL) signaling pathway, which is involved in signal transduction within a variety of cells, including the uterus and mammary epithelial cells. The uterus is exposed to IFN-tau and PL, as well as many others hormones including estrogen, progesterone, and placental growth hormone. The PL stimulates the formation of STAT5 homodimers, which in turn induce the transcription of the bovine uterine milk protein (UTMP) and osteopontin (OPN) genes (Spencer and Bazer, 2002; Stewart et al., 2002; Spencer and Bazer, 2004). In previous studies, the present inventors showed that the UTMP (Khatib et al., 2007a) and OPN (Leonard et al. 2005; Khatib et al. 2007b) genes have surprisingly strong effects on milk production and health traits in cattle. Furthermore, the present inventors showed that STAT1—also a member of the IFN-tau and PL signal transduction pathway—is associated with milk composition and health traits (Cobanoglu et al., 2006).
Studies in mouse have shown that STAT5 is involved in both milk production and fertility; STAT5 knockout female mice fail to lactate (Miyoshi et al., 2001). Also, it has been shown that disruption of Stat5 leads to infertility in females as a result of small-sized or a lack of corpora lutea (Teglund et al., 1998). Because the primary source of progesterone is the corpora lutea of the ovary, lack of development of corpora lutea would have significant effects on the establishment of pregnancy.
Given that STAT5A is a member of the IFN-tau and PL signal transduction pathway, which is very important in both milk production and initiation of pregnancy, and that other genes in this pathway have been found to be associated with milk production and health traits, the present inventors investigated if STAT5A variants are associated with milk production and reproduction traits in dairy cattle.